The present invention relates generally to composite structures having a high strength-to-weight ratio and more particularly to a composite structure preferably formed with an expanding film/foam layer and a reinforcing member.
Discussion
Composite panels formed from lightweight materials are commonly used throughout the aerospace industry for secondary structural applications, such as wing trailing edges, empenage close outs, wing-to-body fairings, strut fairings and access doors. Frequently, these panels are formed from sandwich constructions of relatively thin inner and outer layers of thermoset resin/fiber systems separated by a relatively thick layer of honeycomb core material. The skin materials provide strength and the core material provides stiffness to the composite panel. Although these structures are strong enough to support aerodynamic loading, they typically lack sufficient compression strength for the attachment of assembly details, such as access door hinges. Consequently, the core can crush if a fastener is mounted through it and the outer facesheets.
In such cases, the panels are selectively strengthened in the areas where increased compression strength is needed through the use of xe2x80x98hard pointsxe2x80x99. The usual method of forming xe2x80x98hard pointsxe2x80x99 into a panel is through the use of a monolithic cylinder of potting compound that is incorporated into the panel. A typical potting compound is Epocast 1625 A/B by Ciba Specialty Chemicals.
The use of potting compounds, however, has several drawbacks. A first drawback concerns the density of these compounds. Typically, the potting compounds used for the monolithic inserts have a density of about 0.75 to about 2.00 grams/cubic centimeter. In comparison, the density of the honeycomb material is about 0.07 to about 0.25 grams/cubic centimeter. Accordingly, a considerable weight penalty accompanies the use of such potting compounds.
In the aerospace industry, there are design requirements for structures and materials that exhibit energy absorption. Examples of such structures are aircraft tail skids, helicopter landing gear assemblies and landing strut assemblies of interplanetary spacecraft. Generally speaking, it is highly desirable that materials used for absorbing impact energy provide sustained crush resistance over a defined distance of travel or crush stroke and have a high crush strength-to-density ratio. Additionally, the materials used for absorbing impact energy are frequently required to limit the maximum deceleration force during an impact and as such, the initial peak compressive strength exhibited by the panel is highly significant.
Frequently, structures designed for impact energy absorption use a honeycomb core of a lightweight material, such as aluminum, to provide the impact energy absorption. Examples of these cores are Spiralgrid by Alcore Incorporated and Tube-Core by Hexcel Corporation. However, the ratio of crush strength-to-density for these materials is typically low, generally being on the order of approximately 180 psi per pounds per cubic foot. Furthermore, the initial peak compression strength of these materials is not readily adjustable. Generally speaking, where lower initial peak compression strengths are required, a technique of pre-failing is employed wherein the material is loaded to a point beyond the initial peak compression strength.
The present invention preferably provides a composite structure having a high strength-to-weight ratio that can be fabricated at a relatively low cost. A preferred composite structure uses an expanded syntactic film.
The present invention entails a method for forming a structure having an syntactic film layer and a reinforcing layer.
In one form, a preferred embodiment of the present invention provides a composite structure having first and second layers which are wrapped about one another to form a cylinder. The first layer is formed from an expandable film or foaming adhesive. The second layer is formed from a reinforcing member. The second layer is overlaid onto the first layer. The first and second layers are then rolled together in a jelly roll fashion. The roll is then cured to expand the film. The structure may thereafter be incorporated into a panel and drilled to receive a fastener. A method for forming a composite structure from an expandable film member and a reinforcing member is also provided.